The invention disclosed herein is generally related to motorcycles. More particularly, the present invention is related to suspension and steering systems for the front wheel of a motorcycle.
Modern motorcycles are equipped virtually universally with a fork-type front end suspension system. Such a system includes of a pair of telescoping fork tubes which contain internal springs and dampers. The fork tubes are connected through a head assembly to a rotatable steering column which is journalled to the frame of the motorcycle. The axle of the front wheel is journalled to the lower ends of the fork tubes. This simple structure has been a basic element of motorcycle design for almost a century. Nevertheless, the conventional fork design suffers from certain disadvantages which have long been recognized but which have never been completely overcome. For example, there is the well-known tendency of a motorcycle to "dive," or pitch forward and downward, during hard braking of the front wheel. This tendency has been partially but not adequately mitigated by the advent in recent years of complex anti-dive mechanisms, which operate essentially by increasing the compression pressure in the front fork shock absorbers during braking of the front wheel.
Further, there is a fundamental design problem which arises from the fact that the fork tubes must bear sizeable lateral loads and yet also slide smoothly in a telescoping manner during suspension travel. Lateral stresses exist because the fork is necessarily oriented with a "rake angle," i.e., the fork extends at an angle upwardly and rearwardly from the wheel axle to the steering column. A substantial rake angle is necessary and desirable to obtain satisfactory motorcycle steering response, and is also desirable because it renders the fork most effective in absorbing road shocks, which are directed both upward and rearward from the front wheel. However, the weight load of the motorcycle is purely vertical. If the fork is raked, this results in a lateral stress and consequent frictional resistance to suspension travel. This problem is normally resolved by using large-diameter sliding bushings in the telescoping fork tubes to bear such lateral stresses with a minimum of frictional resistance. As a result, however, the shock absorption performance of the fork tube suspension system varies with the weight and shock loads on the motorcycle.
As a general design principle, it is desirable that weight and shock loads on the front wheel be transmitted to and ultimately borne by the heaviest part of the motorcycle, namely the frame/engine structure comprising the central body of the motorcycle. It is also desirable that such loads be transmitted in the simplest and shortest possible manner in order to minimize the weight of the structures which must transmit and bear these loads. It is further desirable that such loads be borne at a point close to the center of gravity of the motorcycle so as to minimize the polar moment of inertia of the motorcycle and thereby minimize the tendency of the motorcycle to respond to road shocks with a forward pitching motion. Finally, it is desirable from the standpoint of stability to have the center of gravity of the motorcycle as low as possible and centered between the wheels.
In view of these design objectives, it will be recognized that one disadvantage of the fork-type suspension system is that that the load path from the front wheel to the body of the motorcycle must pass through the steering shaft and the associated steering head in which the steering shaft is journalled. This results in a load path which is angular and lengthy. It also requires the steering shaft, the steering head and their supporting structures to be considerably stronger and heavier than would otherwise be necessary to simply accommodate the torque necessary to effect steering, i.e. if a the major weight and shock load-bearing function were not also required. The angular load path also results in shock loads being transmitted to a high point on the frame, in the vicinity of the steering shaft, which is sufficiently far removed from the center of gravity of the motorcycle to result in undesirable pitching motion in response to road shocks. This response is in effect due to the large polar moment of inertia that results from absorbing road shocks at a point far from the center of gravity.
There is yet another disadvantage of the conventional telescopic fork suspension. This disadvantage arises from the fact that, because of the rake angle discussed above, the front wheel has a component of motion during suspension travel that is forward and rearward relative to the body of the motorcycle. Consequently, it is necessary to provide a considerable clearance between the front wheel and the body in order to allow for rearward motion of the front wheel in response to road shocks. In some cases this increases the wheel base of the motorcycle beyond that which would otherwise by desirable solely from the standpoint of performance considerations. In other cases it results in the engine and other body components being positioned close to the rear wheel in order to minimize the wheelbase, thereby resulting in the seat of the motorcycle being higher than may be desired.
Accordingly, it is the object and purpose of the present invention to provide an improved suspension and steering system for the front wheel of a motorcycle.
In this regard, it is a more specific object of the invention to provide in a motorcycle a front suspension system that transfers weight and shock loads in a direct path from the front wheel to a point on the motorcycle body that is low and close to the center of gravity of the motorcycle.
It is another object of the present invention to provide a suspension system in which there is maintained a substantially constant suspension geometry throughout the entire range of ordinary suspension travel.
It is also an object of the present invention, for reasons which will be apparent from the discussion below, to provide a front suspension system that is is one-sided; that is, which is positioned to one side of the front wheel, and with the wheel journalled in a cantilevered arrangement to facilitate removal of the front wheel from one side for maintenance or repair.
It is another object of the present invention to provide in a motorcycle a front suspension and steering system that minimizes bump-steer and diving.
It is yet another object of the invention to provide in a motorcycle a front suspension and steering system in which the travel of the front wheel in response to road shock is in a substantially vertical direction, thereby permitting other favorable design changes to be incorporated into the body of the motorcycle which have heretofore been prohibited in conventional designs by the requirements to provide for travel of the front wheel in a fore-and-aft direction.
Additional objects of the present invention include the provision of brake and spring/damper assemblies for the suspension system described below.